Portable backup batteries might not seem like an exciting gadget, but wait until your smartphone or tablet is dead and there isn’t an outlet in sight. Gadget and gear expert Phil Baker looks at the latest innovations and features in backup batteries.

When you’re not able to snap in a second battery or access an outlet to recharge, a backup battery offers the best solution. It’s typically a small, thin box containing a lithium ion battery and circuitry with a battery gauge, an on-off switch and connectors for charging it and charging your devices using separate USB cables.

They’re incredibly useful in a pinch. I carry one to charge my iPhone and Bluetooth headset when I’m on the road and doing lots of calling. It’s a big help when using the GPS function on your smartphone, which can drain a battery in less than three hours. The battery also recharges my phone on a long airline trip when I’m listening to music or watching videos. Some are even powerful enough to charge an iPad.

What to Look for in a Backup Battery:

The batteries are rated by their capacity, in milliamp hours (mAh). Phones typically have batteries with a capacity of 1200 to about 2000 mAh and tablets of about 6000 mAh.

So if you buy a backup battery for an iPhone 5 that has a 1434 mAh battery built in, then to completely recharge the phone from empty requires a battery of about 1700 mAh. It needs this extra capacity because there is about a 20 percent loss when charging from an external battery.

The maximum output current of the battery is also important, because some batteries can only output a maximum of 0.5 amp (the same as many computers’ USB ports). Others can output up to 2 amps. This affects the ability to charge iPads and iPhones in a reasonable period of time.

The Mophie is an attractive matte black rectangular slab that’s styled to look a little like an iPhone, with a silver band running around the edge. While it may be good-looking, it has some serious deficiencies. It’s unable to be recharged while it’s charging your device, a huge inconvenience. Its on-off switch can be inadvertently turned on, and it requires a separate USB charger and micro USB cable to charge it.

The ZaggSparq 3100 is a slightly larger device and not quite as stylish, but offers many improvements over the Mophie. It can be charged while it is charging, it needs no extra charger or cable, and it plugs directly into an outlet using its built-in folding plug. It can also output sufficient current to charge an iPad.

By Phil Baker for PeterGreenberg.com. Phil Baker has more than three decades of experience in consumer and computer technology product development and program management. Check out his blog at www.philipgbaker.com.